Stem cells have been considered as potential treatments for debilitating diseases of various etiologies, including diabetes, Parkinson's disease and cardiovascular disease. Thus, a critical goal is to define the spectrum of stem cell types displaying characteristics advantageous for the treatment of selected disorders. While the pluripotency and self-renewal of embryonic stem cells is well-recognized, the potential of adult and fetal stem cells has been appreciated only recently.
Stem cells have the potential to develop into many different cell types in the body. Stem cells can theoretically divide without limit to replenish other cells. When a stem cell divides, each new cell has the potential to either remain a stem cell or become another type of cell with a more specialized function, such as a muscle cell, a red blood cell, or a brain cell. Stem cells are often classified as totipotent, pluripotent, and multipotent. Totipotent stem cells (e.g., a zygote) give rise to both the fetus and the extraembryonic tissues. Pluripotent stem cells can give rise to any type of cell except for the extraembryonic tissues (e.g., placenta). Multipotent stem cells can give rise to two or more different cell types but only within a given organ or tissue type. In contrast to stem cells, progenitor cells are unable to self-renew and they give rise to only a few cell types.
A central dogma in embryonic development is that cells undergo a process of fate restriction and commitment. This process begins with the development of the blastocyst; a structure composed of an outer trophoblast layer and an undifferentiated inner cell mass (ICM).
The ICM is the source of embryonic stem cells (ES cells), which are regarded as the quintessential stem cell population (Evans and Kaufman, 1981; Martin, 1981). ES cells demonstrate long-term self-renewal and differentiate into multiple cell types in vitro and in vivo (Smith, 2001; Thomson et al., 1998; Bradley et al., 1984; Amit et al., 2000).
Due to their remarkable in vitro and in vivo plasticity, ES cells have been regarded as the “gold standard” for cell replacement therapy and regenerative medicine. Although the therapeutic potential of ES cells is promising, a number of issues must be addressed prior to clinical use. Ethical concerns and in some cases governmental policies restrict the isolation and cultivation of human ES cells. From a safety perspective, ES cells often form tumors following transplantation into rodents (Evans and Kaufman, 1983). Furthermore, ES cells might not be able to overcome the immunological incompatibility that exists between host and grafted cells (Keller, 2005). With all these unresolved issues many investigators have turned to adult and fetal tissue in search of less controversial stem and precursor populations.
The wide distribution and plasticity of adult stem cells has only recently been appreciated. In addition to the well known stem cells of the adult marrow lymphohematopoietic (Shizuru et al., 2005; Krause et al., 2001) and stromal mesenchymal lineages (Prockop, 1997; Jiang et al., 2002), adult stem cells have been identified in fat (Zuk et al., 2001), liver (Theise et al., 1999), muscle (Lee et al., 2000), and the central nervous system (Reynolds and Weiss, 1992; Morshead et al., 1994; Doetsch et al., 1999) and skin (Toma et al., 2001). Recent reports suggest that the differentiation of adult stem cells is not restricted to derivatives of the tissue in which they reside. Landmark studies have demonstrated that adult stem cells can differentiate into progeny of other embryonic germ layers, a process termed transgerminal differentiation. For example, ectodermal neural stem cells can differentiate into mesenchymal derivatives, including blood (Bjornson et al., 1999), muscle (Galli et al., 2000) and endothelial cells (Wurmser et al., 2004). The plasticity exhibited by adult stem cells has provided hope for the development of new autologous cellular therapies.
Adult stem cells may offer advantages over ES cells; however, their potential use in cell replacement therapies is not without obstacles. Many reports have questioned the plasticity of adult stem cells. Several studies have suggested that in vitro and in vivo transgerminal plasticity is the result of cell fusion rather than actual differentiation (Terada et al., 2002; Ying et al., 2002; Wang et al., 2003). In contrast, others have confirmed in vivo transdifferentiation of marrow cells in the absence of cell fusion (Tran et al., 2003; Pochampally et al., 2004; Sato et al., 2005). It has also recently been demonstrated that MSCs transplanted into the adult brain fail to survive. Even more disconcerting, these cells transferred their cellular labels (bromodeoxyuridine and bis benzamide) to endogenous glia and neural cells, giving a false representation of donor cell plasticity (Coyne et al., 2006; Burns et al., 2006). These apparent contradictory results have raised important issues concerning the nature of adult stem cell plasticity and the broader therapeutic potential they represent.
Fetal stem cells may offer a number of therapeutic advantages over ES and adult stem cells, making them well suited for cell replacement therapy. Fetal stem cells are easily accessible from extra-embryonic tissue that is normally discarded at birth, including the umbilical cord (Nakahata and Ogawa, 1982; Knudtzon, 1974) and placenta (Kaviani et al., 2002; Yen et al., 2005), circumventing many of the ethical concerns presented by ES cell research. Fetal stem cells grow rapidly in culture and exhibit plasticity similar to ES cells, but without documented tumor formation in vivo (Miki et al., 2005). Moreover, fetal stem cells might be more amenable to transplantation due to their immunoprivileged characteristics (Li et al., 2005; Kubo et al., 2001).
The present invention describes the identification and characterization of a fetal stem cell population isolated from explants of amniotic membrane. These amnion-derived stem cells (ADSCs) fulfill all criteria of a stem cell population, including clonality, which has proven difficult in previous studies of putative fetal stem cells (Woodbury et al., 2006; Miki et al., 2005).